One of the most pervasive problems afflicting mankind throughout the world is poor access to clean freshwater and sanitation. With the growing demand for high quality water, many new technologies of water purification are being developed to cater for potable and non-potable use. The reliability and ease of operation of membrane-based filtration systems have led to their proliferation in wastewater treatment. Ultrafiltration (UF) is one such well-developed, low pressure membrane separation process that has been used in different applications, such as water/wastewater treatment, reverse osmosis pre-treatment and separations in the food, dairy, paper, textile, pharmaceuticals, chemical and biochemical industries. However, membrane fouling remains an unavoidable problem in all pressure driven membrane processes, causing deterioration of the membrane performance. These problems generally originate from the accumulation of organics in the effluent water, which serves as a support for the attachment and growth of microorganisms onto the membrane surface. The resulting effects of membrane fouling such as flux reduction, rejection impairment and membrane breakage lead to high operational costs and short replacement intervals.
There are several cleaning methods—such as physical (backwash) and chemical (chemical cleaning) methods available to regenerate the membrane. However, such methods often require high energy consumption and more chemical usage. These methods are ineffective while cleaning biofouling because extra cellular polymeric substances are chemically bound to the membrane surface and hence, it is the most difficult type of fouling to clean. Preventing biofouling on the membrane surface is always better than cleaning the membrane that has been fouled. Membrane materials are more sensitive to fouling and hence, development of new membrane materials or modification of the current membrane materials/membrane surface has constantly been explored to address the problem of fouling.
Polymeric materials based membranes are most widely used for water treatment. This is due to their increased in separation efficiency and decreased maintenance costs when compared to inorganic materials based membranes. Long term liquid based pressure driven separation processes such as ultrafiltration (UF) and microfiltration (MF) requires polymeric membrane materials that possess properties such as good mechanical strength, thermal stability and excellent chemical resistance to have a reduced maintenance cost. Polymeric materials that are commonly used include polysulfone (PS), cellulose acetate (CA), polyvinyl alcohol (PVA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), cellulose nitrate (CN), polyethersulphone (PES), poly acrylonitrile (PAN), etc. In recent years, metal organic framework based (MOF) membranes, block copolymers based membranes, carbon nanomaterials based membrane and aquaporin based bio-mimetic membranes are getting much attention in the field of desalination and water treatment.
In recent years, graphene also gained much attention in the field of membrane science and engineering due to its high surface area, mechanical strength and chemical stability. Theoretical analysis has also predicted that graphene-based membranes may exhibit 2-3 orders of magnitude higher permeability than the current state of the art membranes. However, experimental studies show that there are limitations in achieving such improved permeability due to the challenges associated with the fabrication of leak-free porous graphene membranes with very large surface area.
In recent years, carbon nanotubes and graphene have been viewed as new types of nanofillers to improve the selectivity and the separation performance of polymeric ultrafiltration (UF) membranes due to the interaction of contaminants with the delocalized p-electrons of the nanocarbons. It is also reported that the nanocarbon based fillers enhance the thermal stability and mechanical strength of the polymeric membranes. However, it is commonly accepted that supported graphene is hydrophobic in nature and its water contact angle is, to some extent, higher than that of graphite. Moreover, these fillers are normally associated with relatively poor dispersion within the polymer matrix, a problem which limits the antifouling property of the membrane.
Attempts have been made to produce graphene-based membranes.
US 2015/0053607 A1 discloses a graphene derivative composite membrane and a method for fabricating the same. The graphene derivative composite membrane comprises a supporting membrane made of porous polymer and a plurality of graphene derivative layers disposed on the support membrane. The graphene derivative and the polymer form different layers of the membrane.
US 2012/0255899 A1 discloses a graphene-containing separation membrane. The separation membrane includes a polymer support and graphene on at least one surface of the polymer support. The graphene may be in direct contact with the polymer support or an intermediate layer may be provided between the polymer support and the graphene. The graphene and the polymer form different layers of the membrane.
Membrane that is made of different layer may involve highly complex membrane fabrication process, which requires many steps, and the possibility of membrane failure owing to delamination of the separation layer, which occurs during sintering or variations in filtration pressures. Such membrane is usually used only in specific applications based on the properties of the materials used in forming the different layers.
Graphene oxide is an amphiphilic graphene derivative (a single-atom layer of graphene) with oxygen containing functional groups (—OH, —COOH) attached to both sides of the graphene flake. With the addition of the oxygen groups, the double bonds holding the carbon atoms together can break more easily, resulting in the loss of the material's electrical conductivity. However, there are important beneficial aspects to having these oxygen containing functional groups attached to the graphene structure. Firstly, they impart polarity to the flake so that the graphene oxide is able to be more uniformly dispersed in solvents and eventually in the polymer matrix it is blended into. Further, these oxygen containing hydrophilic functional groups improve the wetting properties (hydrophilic properties) of the normally hydrophobic polymeric membranes through hydrogen bonding. Indeed, literature reports that the contact angle of water on pristine graphene oxide films can vary from 0 (by theory) to 60 (by experiment) degrees.
Even though graphene and its derivatives have been shown to exhibit the potential for better performance, the use of these material based membranes in real applications is still a dream due to their hydrophobicity and limitations in the fabrication process.
It is therefore desirable to provide a membrane and a method of producing the same that seek to address at least one of the problems described hereinabove, or at least to provide an alternative.